1. Field of the Invention
The present invention relates to a divided photodiode incorporated into a light-receiving element used for an optical pickup device or the like. More particularly, the present invention relates to a divided photodiode having a structure which can provide an improved response speed therefor.
2. Description of the Related Art
An optical pickup device is used for various types of optical disk apparatus including those used for a CD-ROM, a DVD and the like. In recent years, a DVD, in particular, has been developed more and more actively and rapidly. Such optical disk apparatus are now required to process a large amount of data necessary for storing a moving picture and the like. In addition, it is very probable that a DVD having a response speed twice or even four times as fast as that of a currently available DVD will be developed in the near future. In view of these circumstances, there is a strong demand for realizing an even higher response speed of an optical pickup device.
A divided photodiode element, in which a light-receiving region is divided into a plurality of light detecting sections, has conventionally been used as a signal detector element for an optical pickup device.
As high-performance optical disk apparatus of smaller size have been realized in recent years, it has become increasingly important to reduce the size and weight of an optical pickup device. In order to realize such an optical pickup device, an optical module including a single hologram element, into which a tracking beam generation portion, a light branching portion and an error signal generation portion have been integrated, has been proposed. The optical module is provided on a top surface of an integrated single package incorporating a laser diode, a photodiode and the like in the inside thereof.
FIG. 18 shows a schematic arrangement of an optical system for an optical pickup device including such an optical module.
Hereinafter, the signal detection principle of the optical system will be briefly described. Light emitted from a laser diode LD is split by a tracking beam generating diffraction grating 30, which is formed under the lower surface of a hologram element 31, into three optical beams. i.e., two sub-beams used for tracking and one main beam for reading out an information signal. Then, the light beams are transmitted through the hologram element 31 formed on the upper surface of the package as zero-order light, transformed by a collimator lens 32 into parallel light beams and then converged by an objective lens 33 onto a disk 34.
The light is reflected from the disk 34 with being modulated by the pits formed on the disk 34, transmitted through the objective lens 33 and the collimator lens 32, and then diffracted by the hologram element 31 so as to be guided as first-order diffracted light beams onto a five-divided photodiode PD, on which five divided light-detecting sections D1 to D5 (hereinafter, also referred to as "light-detecting photodiode sections D1 to D5") are formed.
The hologram element 31 includes two regions 31a and 31b having respectively different diffraction periods. When the reflected light of the main beam is incident onto one of the two regions, the light is converged onto an isolating section which isolates the light-detecting sections D2 and D3 from each other. On the other hand, when the reflected light of the main beam is incident onto the other region of the hologram element 31, the light is converged onto the light-detecting section D4. The reflected light beams of the two subbeams are converged by the hologram element 31 onto the light-detecting sections D1 and D5, respectively.
In this optical system, the incidence positions of the reflected main beams on the photodiode PD are moved along the longitudinal direction of the pair of light-detecting photodiode sections D2 and D3 in accordance with the variation of a distance between the hologram element 31 and the disk 34. In the case where the main beam is in focus on the disk 34, the reflected light beam thereof is incident onto the isolating section between the pair of light-detecting sections D2 and D3 of the photodiode PD.
Thus, assuming that the outputs of the light-detecting sections D1 to D5 of the five-divided photodiode PD are denoted by S1 to S5, respectively, a focus error signal FES is given by the equation: FES=S2-S3.
On the other hand, a tracking error is detected by a so-called "three-beam method". Since the two subbeams for tracking are converged onto the light-detecting sections D1 and D5, respectively, a tracking error signal TES is given by the equation: TES=S1-S5. Thus, when the tracking error signal TES is zero, the main beam is correctly located on the target track which is intended to be irradiated with the main beam.
Furthermore, a reproduced signal RF is given as a sum of the outputs of the light-detecting sections D2 to D4 for receiving the reflected light of the main beam, based on the equation: RF=S2+S3+S4.
FIG. 19 is a cross-sectional view taken along the line a-a' of the conventional light-detecting divided photodiode shown in FIG. 18. It is noted that various components including multi-layer wires, protective films and the like to be formed during the respective process steps succeeding a metal wire processing step are omitted in FIG. 19. In FIG. 19, D1, D2, D3 and D5 denote the light-detecting sections.
Hereinafter, a method for fabricating the divided photodiode will be described with reference to the cross-sectional views shown in FIGS. 20A and 20B. In FIG. 19 and FIGS. 20A and 20B, the same components are identified by the same reference numerals.
First, as shown in FIG. 20A, P-type isolating diffusion regions 2 are formed in the regions in a P-type semiconductor substrate 1 which are to be the isolating sections for isolating the light-detecting sections D1 to D5 from each other.
Next, as shown in FIG. 20B, an N-type epitaxial layer 4 is formed over the entire surface of the P-type semiconductor substrate 1. Then, P-type isolating diffusion regions 5 are formed in the regions, corresponding to the respective P-type isolating diffusion regions 2, in the N-type epitaxial layer 4. These P-type isolating diffusion regions 5 are formed so as to vertically extend from the surface of the N-type epitaxial layer 4 to reach the upper part of the P-type isolating diffusion regions 2. In other words, each pair of P-type isolating diffusion regions which consists of the regions 2 and 5 is formed so as to range from the surface of the N-type epitaxial layer 4 to the surface of the P-type semiconductor substrate 1. As a result, the N-type epitaxial layer 4 is divided into a plurality of (four, in the example shown in FIG. 20B) electrically isolated N-type semiconductor regions, so that the respective light-detecting sections D1 to D5 are formed (though the light-detecting section D4 is not shown in FIG. 20B).
Next, N-type diffusion regions 6 are formed in the respective sections of the divided photodiode in the surface of the N-type epitaxial layer 4. The serial resistance of the photodiode is reduced by these N-type diffusion regions 6. As a result, a CR time constant thereof is reduced and thus high-speed response characteristics are realized.
Thereafter, as shown in FIG. 19, an oxide film 12 having through holes is formed on the N-type epitaxial layer 4 including these N-type diffusion regions 6, and electrodes 13 are formed on the oxide film 12. In this manner, the conventional light-detecting divided photodiode shown in FIG. 19, in which each of the electrodes 13 is electrically connected to an associated one of the isolating diffusion regions 5 via an associated one of the through holes, can be formed.
A high-speed operation is required for the light-detecting sections D2, D3 and D4 for processing the reproduced signal RF. In the case where a light beam is irradiated onto the isolating section between the light-detecting sections D2 and D3, the light-detecting sections D2 and D3, in particular, are required to be operated at an even higher speed. However, in the case where a light beam is irradiated onto the isolating section between these light-detecting sections D2 and D3, the cutoff frequency of the divided photodiode PD is decreased as compared with the case where a light beam is irradiated onto the center of the light-detecting section D2 or D3. Herein, the cutoff frequency means a frequency at which a gain is decreased by about 3 dB as compared with a gain in a low frequency region.
The experimental results demonstrating the decrease in cutoff frequency of the divided photodiode PD are shown in FIGS. 21A and 21B.
FIG. 21A is a cross-sectional view showing the vicinity of the pair of light-detecting sections D2 and D3 of the divided photodiode PD shown in FIG. 19. On the other hand, FIG. 21B is a graph showing the dependence of the cutoff frequency of the divided photodiode PD upon the position of a light beam. In FIG. 21B, the axis of abscissas represents the positions of the light beams of the diffracted light which have been incident in the vicinity of the light-detecting sections D2 and D3, and the axis of ordinates represent the cutoff frequencies fc (MHz) at the respective positions. The measurement results shown in FIG. 21B are obtained under the conditions where the specific resistance of the P-type semiconductor substrate 1 is set at about 15 .OMEGA.cm, the reverse bias applied to the photodiode is set at about 1.5 V and the load resistance is set at about 380.OMEGA..
As can be understood from FIG. 21B, when the incident light beam is located in the vicinity of the isolating section between the pair of light-detecting sections D2 and D3, the cutoff frequency fc is decreased as compared with the case where the light beam is located in the center of the light-detecting section D2 or D3. In the case where the light beam is incident onto the isolating section between the light-detecting sections D2 and D3, the cutoff frequency has a value slightly larger than 20 MHz. Thus, the photodiode having such a cutoff frequency can be adapted to a DVD. However, the photodiode cannot be operated with an even higher response speed for a double-speed DVD, a quadruple-speed DVD or the like.
The cutoff frequency is decreased when a light beam is incident onto the isolating section between the light-detecting section D2 and D3, because the optical carriers, which have been generated in the region of the P-type semiconductor substrate 1 under the P-type isolating diffusion region 2, make a detour around the P-type isolating diffusion region 2 to reach a depletion layer formed in a P-N junction between the N-type epitaxial layer 4 and the P-type semiconductor substrate 1. More specifically, since the optical carriers, which have been generated under the P-type isolating diffusion region 2, are required to diffusively move over a distance of about several tens of .mu.m. Such a long movement distance decreases the cutoff frequency of the photodiode.
FIG. 22 shows the results of the simulation performed for obtaining current paths in the P-type isolating diffusion region 2 and in the vicinity thereof corresponding to the isolating section between the light-detecting sections D2 and D3, in which the directions of the current are indicated by arrows. In FIG. 22, the position represented by an ordinate of 0 .mu.m corresponds to the surface of the substrate, and the lower end of the P-type isolating diffusion region 2 is located under the surface of the substrate.
The electrons functioning as optical carriers move in the opposite directions to those indicated by the arrows in FIG. 22. As can be understood from FIG. 22, the optical carriers make a detour around the P-type isolating diffusion region 2 functioning as the isolating section to reach the depletion layer existing in the P-N junction formed between the N-type epitaxial layer 4 and the P-type semiconductor substrate 1.
FIG. 23 is a graph showing the potential distribution in the isolating section between adjacent light-detecting sections in the depth direction. In FIG. 23, the axis of ordinates represents a potential (Volts) and the axis of abscissas represents a depth (.mu.m) from the surface of the substrate. The region 5 corresponds to the P-type isolating diffusion region 5 and the region 2 corresponds to the P-type isolating diffusion region 2.
As can be understood from FIG. 23, in this potential distribution, the P-type isolating diffusion region 2 has a large potential, thereby functioning as a potential barrier against the electrons which are the optical carriers moving in the substrate 1 toward the surface thereof. Therefore, as shown in FIG. 22, the optical carriers move while making a detour around the P-type isolating diffusion region 2.
Typically, the specific resistance of a commonly used P-type semiconductor substrate 1 is about 15 .OMEGA.cm. Thus, in the case where the reverse bias of about 1.5 V is applied to the light-detecting photodiode sections constituting the respective light-detecting sections is about 1.5 V, the distance over which the optical carriers run while making a detour becomes about several tens of .mu.m as shown in FIG. 22.
In order to solve the above problems, various measures have been taken.
For example, a divided photodiode having such a structure as that shown in FIG. 24 is suggested in Japanese Patent Application No. 8-166284 (corresponding to Japanese Laid-Open Publication No. 9-153605).
The divided photodiode shown in FIG. 24 uses a substrate having a high specific resistance as the P-type semiconductor substrate 1, unlike the conventional divided photodiode shown in FIG. 19. Thus, when a reverse bias at an equal level is applied to the photodiodes shown in FIGS. 19 and 24, the area of the depletion layer 21 expanding in the P-N junction between the N-type epitaxial layer 4 and the P-type semiconductor substrate 1a in the photodiode shown in FIG. 24 becomes larger as compared with the photodiode shown in FIG. 19. Accordingly, the depletion layer expands to a larger degree toward the region below the P-type isolating diffusion region 2 located in the isolating section between the light-detecting sections D2 and D3. As a result, the distance over which the optical carriers generated in the P-type semiconductor substrate 1 under the P-type isolating diffusion region 2 run while making a detour around the P-type isolating diffusion region 2, is shortened. Consequently, the response speed and the cutoff frequency of the photodiode are increased. The higher the specific resistance of the substrate is set to be, the shorter the running distance of the optical carriers can be and the higher the response speed of the photodiode can be.
However, as a result of more detailed researches, the present inventors found that the response speed cannot always be satisfactorily increased merely by increasing the specific resistance of the substrate.